Definition of the word cis

The word "cis" in the context of chemistry refers to the location of functional groups on a molecule that are directly attached to the carbon atom of a double bond or a ring. The term "cis" is derived from the Latin prefix "cis," which means "on this side" or "this side of." In organic chemistry, compounds can be classified as being either "cis" or "trans" based on the geometric isomerism of the molecule, which refers to the spatial arrangement of the functional groups. When the two functional groups are located on the same side of the double bond or ring, the molecule is called "cis." For example, in 1,2-dichloroethene, the two chlorine atoms are on the same side of the double bond, making it a cis compound. In contrast, when the functional groups are located on opposite sides of the double bond or ring, the molecule is called "trans." For instance, in 1,2-dichloroethene, the trans compound would have the chlorine atoms on opposite sides of the double bond. The terms "cis" and "trans" provide a useful way to differentiate between geometric isomers and to predict their properties based on their structures. Cis isomers generally have shorter bonds and lower barriers to internal rotation, which can lead to different reactivity and physical properties compared to trans isomers. Overall, understanding the origin and meaning of "cis" is an important concept in organic chemistry, as it helps to characterize and classify complex molecules based on their structures.